main-toolbar: Guard against being in a transitional state

We going back from the preview, we first unset the active item in
PhotosItemManager, and then change the window mode in
PhotosModeController. If we ever reach this code in the middle of this
transition, we should not crash.

Fixes: https://bugzilla.gnome.org/726519
main-toolbar: Remove useless signal connections

We do not use PhotosSourceManager or any of the sources in it while
setting the title. Hence there is no need to listen for the active
source.

Fixes: https://bugzilla.gnome.org/726519
main-toolbar: Remove empty else branch

The else branch was left empty waiting for PhotosSearchTypeManager and
PhotosSearchController to be implemented so that we can change the
title to "Albums" or "Results for 'search string'". This should not be
done in the new split view designs.

Fixes: https://bugzilla.gnome.org/726519